Both the first marsupial and the first placental have been found in China.
As I said, because of their small size, fossil mammals are more often known from isolated
teeth than from complete skeletons, but fortunately exceptions exist that allow us to understand
the main steps of their evolution. Here is a reconstruction of the oldest placental
known to date. Its name is Juramaia, and the anterior part
of its skeleton has been found in the Middle Jurassic of China.
It was a small animal, not exceeding 20 cm in total length, but it demonstrates that
modern types of mammals appeared some 165 million years ago.
However, it would be wrong to think that more primitive mammals, without living relatives,
did not evolve during the age of dinosaurs. Again, some more or less complete fossils
reveal the existence of surprisingly specialized stem-mammals.
Fruitafossor for example is a fossorial stem-mammal, the lineage of which appeared well before
the separation between the placental and marsupial lineages.
A relatively complete skeleton of this little animal, the weight of which did not exceed
6 grams, was found in the Late Jurassic of North America.
Its peg-like teeth devoid of enamel are very similar to those of a modern armadillo, indicating
a specialized diet made of colonial insects. It possessed also large front limbs, well
adapted for digging, so that it was able to break into termite mounds.
Although primitive, Fruitafossor shows therefore a high degree of specialization, both for
digging and in terms of diet, challenging the idea that early mammals were restricted
to unspecialized niches. Another good example of a specialized stem-mammal
is the gliding Volaticotherium from the Middle Jurassic of Asia.
Its gliding membrane, or patagium, was supported by the limbs and the tail.
It is the first known gliding mammal, although it is only distantly related to modern flying
squirrel. The shape of its teeth indicates that it was
a specialized insect eater. Castorocauda is a semi-aquatic stem-mammal
with a broad tail similar to that of a modern beaver.
It was first found in the Middle Jurassic of China and is therefore some 165 million
years old. Fossilized impressions of some webbing are
present between the toes, reinforcing the idea it was well-adapted to an aquatic mode
of life. Its sharp teeth also suggest it was piscivorous,
eating mainly fish. It is the very first mammal adapted to a semi-aquatic
mode of life in the fossil record, and such a specialization will not reappear in mammals
before the Eocene, 56 million years ago. The forelimbs of Castorocauda were robust
and adapted for digging as well. They are quite similar to those of the modern
Platypus, which both digs and swims. However, this is again a convergence as Castorocauda
is a stem-mammal only distantly related to Platypus.
Another interesting feature about this critter is its size.
It could grow longer than 40 cm and it is actually the largest Jurassic mammal known
to date. Repenomamus from the Early Cretaceous of China
is about 125 million years old. It is thus younger than Juramaia, Fruitafossor,
Volaticotherium or Castorocauda. It could exceed 1 m in length and its overall
appearance is quite reminiscent to that of a modern Tasmanian devil.
One fossil specimen displayed some remains of a juvenile of the small ornithischian dinosaur
Psittacosaurus at the level of its stomach. Repenomamus was therefore an active predator,
which may also have scavenged dinosaur carcasses. It is the largest known Mesozoic mammal, and
it was indeed larger than some of the predatory theropod dinosaurs that could be encountered
at the same time in China. So, the very first mammals evolved in the
Late Triassic, at the same time as the dinosaurs, from one of the synapsid lineages that survived
the end-Permian mass extinction. However, the rather arid climate at the time
was more beneficial to dinosaurs than to mammals and the former quickly outcompeted mammals
and reached dominating position in the Late Triassic ecosystems.
We shall see, however, in the next video that it was not so easy for the dinosaurs.
That does not mean, however, that mammals did not evolve during the age of dinosaurs.
It is often said that Mesozoic mammals were all small nocturnal mouse-like animals.
This is clearly an oversimplification. Even if mammals did not reach dominant positions
in Mesozoic ecosystems before the demise of dinosaurs at the end of the Cretaceous, they
nevertheless diversified during the Mesozoic, becoming gliding, digging or aquatic.
Most of the actors of this first diversification event are not directly related to modern mammals
though. The first representatives of the two main
modern lineages, the marsupials and the placentals, appeared anyway as early as the Middle Jurassic,
165 million years ago. The Late Triassic was also the time when many
other lineages that characterize our modern vertebrate faunas appeared.
Among them, we found the lizards. The first representative of the lizard and
snake lineages, the Lepidosauromorpha appeared in the Carnian, 225 million years ago, and